well, either the contest participants don't care what the prize is or they like the prizes being offered because there are loads more contestants every month.
i also think they should keep it at 4 (or more) winners because there are so many different styles of art being entered (video, drawing, models, cakes, knitting etc.) and so many good entries of all levels that it is very difficult for Aardman staff to narrow down the choices to just 3.
i agree with jills, ptmwc, that you have a valid point but i don't think that's what Aardman's intentions were when they decided to host the contest. makes for a good discussion however.